This invention relates to improvements in an oil pump driven by an engine, and more particularly to the improvements in the oil pump which is provided with a function of decreasing a pulse pressure in a discharge port.
An oil pump of the type driven by an engine is disclosed in Japanese Utility Model Provisional Publication No. 2-43482. This oil pump is of a trochoid type and has a basic arrangement such that volumes of a plurality of pump chambers formed between an inner rotor and an outer rotor are continuously changed to increase and decrease under a driving force received from an engine, in which oil sucked in a suction port is pressurized in the pump chambers and discharged to a discharge port whose upper section of the discharge port has a closed groove whose upper portion of the closed groove is formed into an air chamber where air is accumulated.
Since this oil pump is provided with the air chamber as the closed groove in communication with the discharge port, a plurality of the pump chambers sequentially open to the discharge port so as to discharge oil to the discharge port, generating pulse pressure. This pulse pressure can be absorbed under a dumping action of the air chamber.
However, in case of this conventional oil pump, when the frequency of the pulsation pressure in the discharge port becomes in arrangement with the resonance frequency of the air chamber, vibration within the air chamber increases, and then air within the air chamber may be rapidly discharged to the discharge port. Air is thus leaked under resonance of the air chamber so that a capability of reducing pulse pressure is rapidly lowered. This affects an actuator and the like driven by the discharged oil. Additionally, rapid change of noise level provides discomfort to passengers. More specifically, the passengers do not sense much discomfort when the noise level of the pump is linearly increased almost in proportion to an engine speed; however, the passengers sense much discomfort when the noise level of the pump is rapidly changed during the rise of the engine speed.